Fatty ester compositions with improved oxidative stability

ABSTRACT

Compositions containing unsaturated fatty esters may be stabilized against atmospheric oxidation by the addition of an antioxidant package containing at least one nitroxide free-radical scavenger and at least one alkylalkanolamine. Compositions treated in this manner show good resistance to atmospheric oxidation and resultant viscosity increase. An advantage of the nitroxide free-radical scavenger is that it stops the oxidation of the unsaturated fatty esters already during the initiation stage. Moreover, it is much less volatile than for example the known alkylhydroxylamine oxygen scavengers. By the use of a nitroxide free-radical scavenger, the composition can thus be stabilized for a longer period of time. The stability period is moreover less affected by the supply of oxygen to the composition. The solubility problem of the nitroxide in the fatty ester component can be solved by dissolving the nitroxide first in the alkylalkanolamine.

The invention relates to fatty ester compositions. More particularly, it relates to fatty ester compositions that contain at least 50 wt % of a fatty ester component which comprises an unsaturated fatty ester and that contain moreover additives, including at least one alkylalkanolamine, that reduce their oxidative degradation.

BACKGROUND OF THE INVENTION

Fatty esters are widely used commercially in a variety of applications. Commonly used esters include natural fats and oils, especially triglyceride oils. Well known examples include soybean oil, rapeseed oil, jathropa oil, palm oil, canola oil, olive oil, linseed oil, and tung oil.

Another important type of fatty ester is biodiesel, a clean-burning alternative fuel produced from domestic, renewable resources. Biodiesel contains no petroleum, but it can be blended at any level with petroleum diesel to create a fuel blend. It can be used in compression-ignition (diesel) engines with little or no modification. Biodiesel is biodegradable, essentially nontoxic, and essentially free of sulfur and aromatic compounds, and thus can provide certain environmental advantages. Biodiesel is more particularly an alkyl ester fuel that meets the specifications of ASTM D 6751, which is incorporated herein by reference.

Biodiesel is essentially a mixture of methyl, ethyl, and/or isopropyl esters of fatty acids, made through transesterification of fatty acid triglycerides (oils) with the respective alcohols. The most commonly used raw material oils are seed oils such as soybean oil, palm oil, and rapeseed oil.

These and many naturally occurring fats and oils contain a component, sometimes a major one, of unsaturated fatty acids (mainly in the form of esters). These include such acids as oleic, linoleic, linolenic, and others bearing one or more olefinic moieties. Accordingly, biodiesel fuels made from these oils also typically contain unsaturated acids and/or esters thereof. In both natural oils and biodiesel, the unsaturation makes the materials susceptible to oxidation by atmospheric oxygen and perhaps other oxidants. Such oxidation, for example during processing or storage, may result in an increase in rancidity, viscosity and/or pour point temperature, which in many cases is undesirable. Therefore, ways of reducing or eliminating oxidative degradation of fatty esters are sought in the various industries in which these materials are used.

WO 2007/146567 discloses an alkyl ester fuel, in particular a biodiesel, the oxidative stability of which is improved by adding a combination of an alkylalkanolamine and an alkylhydroxylamine. Both additives, and the combination thereof, were found to increase the oxidative stability. In the examples relatively high amounts of both additives were used, namely 500 ppm of DEHA (diethylhydroxylamine) and 1000 ppm of ODEA (octydiethanolamine).

In the fatty ester compositions disclosed in US 2007/0137098, use is also made of an alkylhydroxylamine (in particular DEHA) to improve the oxidative stability thereof. The alkylhydroxylamine is more particularly used in combination with a phenolic antioxidant. This combination appeared to have synergetic effects on the oxidative stability of the fatty ester composition. The phenolic antioxidant is in particular a hydroquinone which is commonly used in biodiesel compositions. In the examples, the hydroquinone (phenolic antioxidant), which is a quite expensive additive, was used in relatively small amounts of 5 to 100 ppm. Notwithstanding the observed synergetic effects, the DEHA (alkylhydroxylamine) was however still to be used in quite high amounts of 10 000 to 40 000 ppm.

The oxidative stability of biodiesel is an important property thereof and is subjected to criteria which may differ from country to country. In the US, oxidative testing of biodiesel is done according to ASTM 6751-08 whereby the B100 (100% biodiesel) must pass a minimum of three hours oxidative stability as done by the Rancimat test. The European oxidation criteria is a minimum of six hours as per EN 14112. Some commercial biodiesel manufacturers and blenders require even eight hours oxidative stability to ensure that B100 biodiesel stored for prolonged periods of time will be suitable for commerce.

A problem with the biodiesel compositions disclosed in WO 2007/146567 and in US 2007/0137098 is that their long term oxidative stability depends to a large extent on the conditions wherein the biodiesel is handled and stored. The antioxidants used therein are indeed oxygen scavengers which decompose or are converted to their oxidized form by reaction with oxygen. They react with the oxygen dissolved in the biodiesel and, when they are rather volatile like DEHA, they may even react with the oxygen present in the headspace above the biodiesel. When the storage tank is kept closed, the biodiesel can be stored for a long period of time. However, with continued ingress of oxygen into the biodiesel (from opening and closing the storage tank), the antioxidants can be quickly consumed.

An object of the present invention is therefore to provide a new fatty ester composition, which comprises at least one alkylalkanolamine and the oxidative stability of which is less dependent on the amount of oxygen which comes into contact with the composition.

SUMMARY OF THE INVENTION

To this end, the composition according to the invention further comprises at least one nitroxide free-radical scavenger or a precursor thereof.

Oxidation of the fatty ester component follows three steps: initiation, propagation and termination. During the initiation step, free radicals are produced by reaction with initiator molecules. These free radicals can then react with dissolved oxygen producing peroxide molecules and subsequently new free radicals. Continuation of this propagation stage is a chain reaction resulting in numerous hydroperoxides and fatty ester radicals leading to accelerated oxidative instability until the dissolved oxygen is consumed. The advantage of the nitroxide free-radical scavenger used in the composition of the present invention is that it stops the oxidation process already during the initiation stage so that smaller amounts thereof are required and so that the consumption of this free radical scavenger is not, or at least much less, affected by the amount of oxygen which is supplied to the fatty ester composition.

During the oxidation of hydroxylamines as used in US 2007/0137098 and in WO 2007/146567, reactive free radicals may also be formed, as an intermediary product, but these free radicals are only formed during the propagation stage and are not stable and react quickly away. The nitroxide free-radical scavengers used in accordance with the present invention, are on the contrary stable compounds which remain in the composition until they react with any free radical produced during the initiation stage. An important advantage of the nitroxide free-radical scavengers is further that, in contrast to the alkylhydroxylamines used in the prior art, they are also much less volatile so that they remain in the composition.

In the composition according to the present invention, the nitroxide free-radical scavenger is combined with an alkanolamine. A first advantage of this combination is that the nitroxide can easily be dissolved into the alkylalkanolamine whereas it is difficult to be dissolved in the fatty ester component itself. A further advantage of this combination is that the antioxidant effects of the nitroxide are improved by the presence of the alkylalkanolamine, i.e. this combination has a synergetic effect on the oxidative stability of the composition.

The nitroxide free-radical scavenger has preferably the formula (I):

wherein each of R₁, R₂ and R₃ and R₄ is an alkyl group or heteroatom substituted alkyl group having 1 to 15 carbon atoms, and wherein R₅ and R₆ (a) each being an alkyl group having 1 to 15 carbon atoms, or a substituted alkyl group having 1 to 15 carbon atoms wherein the substituent is halogen, cyano, —CONH₂, —SC₆H₅, —S—COCH₃, −OCOCH₃, —OCOC₂H₅, carbonyl, alkenyl wherein the double bond is not conjugated with the nitroxide moiety, or —COOR wherein R of the —COOR group is alkyl or aryl, or (b) together forming part of a ring that contains 4 or 5 carbon atoms and up to two heteratoms of O, N or S.

In an advantageous embodiment of the composition according to the invention, the nitroxide is selected from the group consisting of 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy (4-hydroxy TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidino-1-oxy and 2,2,6,6-tetramethylpiperidino-1-oxy (TEMPO), the nitroxide comprising preferably 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy.

In a further advantageous embodiment of the composition according to the invention, it comprises at least one alkylalkanolamine selected from the group consisting of N-alkylalkanolamines, N-alkyldialkanolamines and N-dialkylalkanolamines, the alkylalkanolamine having preferably the formula (II):

R¹R²NCH₂CH₂OH

wherein R¹ is an alkyl group or an isoalkyl group of 3 to 24 carbon atoms and R² is —H, —CH₂, —CH₂CH₂OH or —R¹.

In still a further advantageous embodiment of the composition according to the invention, it comprises at least one hydroxylamine which has the formula (III):

R¹R²NOH

wherein R¹ and R² are each independently hydrogen, a linear or branched, saturated or unsaturated C1-C20 aliphatic moiety, or a C6-C12 aryl moiety, a C7-C14 araliphatic moiety or a C5-C7 cycloaliphatic moiety.

The composition according to the invention may comprises other additives, in particular:

a) a nonphenolic oxygen scavenger or precursor thereof; wherein the non-phenolic oxygen scavenger or precursor thereof includes a hydroxylamine, an amine N-oxide, an oxime, a nitrone, or a mixture of any of these,

b) a primary, secondary or tertiary un-substituted or substituted amine, and

c) a phenolic oxygen scavenger or precursor thereof, comprising for example an oxidized or reduced quinone, which quinone may be substituted.

The present invention also relates to a method of making a stabilized composition comprising blending together a fatty ester component comprising an unsaturated fatty ester, at least one nitroxide free-radical scavenger or a precursor thereof and at least one alkylalkanolamine.

Advantageously, said nitroxide free-radical scavenger is dissolved in said alkylalkanolamine or in a hydroxylamine or in a mixture of a hydroxylamine with the alkylalkanolamine before being blended with said fatty ester component.

The nitroxide free-radical scavengers described hereabove dissolve only slowly in the fatty ester composition so that this composition should be heated. It has been found that the nitroxide free-radical scavenger can however be dissolved easily in the alkylalkanolamine or in a hydroxylamine (or in a mixture thereof) so that heating and subsequently cooling of the fatty ester composition can be avoided.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, compositions comprising fatty esters may be treated by the addition of an antioxidant package that slows or prevents oxidative instability such as an increase in rancidity as reflected in an increase in viscosity of the composition and/or increases in the pour point temperature. The composition includes at least the following:

a) a fatty ester component constituting at least 50 wt % of the composition and comprising an unsaturated fatty ester;

b) at least one nitroxide free-radical scavenger or precursor thereof; and

c) at least one alkylalkanolamine.

In other to further increase the oxidative stability of the composition, it may additionally comprise:

d) a nonphenolic oxygen scavenger or precursor thereof; wherein the non-phenolic oxygen scavenger or precursor thereof includes a hydroxylamine, an amine N-oxide, an oxime, a nitrone, or a mixture of any of these,

e) a phenolic oxygen scavenger or precursor thereof; wherein the phenolic oxygen scavenger or precursor thereof includes a reduced or oxidized quinone that may be substituted, and

f) a primary, secondary or tertiary un-substituted or substituted amine.

Compositions to be treated with the antioxidant package include those containing at least 50 wt % of a fatty ester component comprising unsaturated fatty esters. Typically the fatty ester component will constitute at least 80 wt % of the composition, more typically at least 90 wt % and most typically at least 95 wt %. The fatty ester may be natural or synthetic. Usually the fatty ester component contains at least 10 wt %, in particular at least 25 wt % and more particularly at least 40 wt % of unsaturated fatty esters. Nonlimiting examples of natural esters include soybean oil, japtropha oil, canola oil, corn oil, olive oil, linseed oil, palm oil, rapeseed oil, safflower oil, sunflower oil, algae and tung oil. In certain embodiments, the ester may be a biodiesel, by which is meant a natural oil that has been transesterified with a lower alcohol, typically methanol, ethanol, and/or isopropanol. Biodiesel derived from any natural or synthetic fat or oil is suitable for treatment according to the invention, for example algae. The composition may also contain a petroleum distillate, or it may be essentially free of distillates.

Petroleum distillates suitable for admixture with biodiesel fuels for use according to the invention include any of a variety of petroleum-based fuels, including but not limited to those normally referred to as “diesel.” Exemplary distillates may also include gasoline, gas-oil, and bunker fuel. Petroleum middle distillates will be used in many applications, and such middle distillates include mineral oils boiling in a range from 120 to 450° C. obtained by distillation of crude oil, for example standard kerosene, low-sulfur kerosene, jet fuel, diesel and heating oil such as No. 2 fuel oil. Exemplary distillates that may be blended with biodiesel for treatment with an antioxidant package of this invention are those which contain not more than 500 ppm, in particular less than 200 ppm, of sulfur and in specific cases less than 50 ppm of sulfur or even less than 5 ppm. Useful distillates, especially middle distillates, are generally those which were subjected to refinement under hydrogenating conditions and which therefore contain only small amounts of polyaromatic and polar compounds that impart natural lubricating activity to them. Distillates that have 95% distillation points of less than 370° C., in particular less than 350° C., and in special cases less than 330° C., may also be used.

Biodiesel is susceptible to oxidative instability due to unsaturation within the methyl esters. Oxidation of B100 follows three steps: initiation, propagation and termination. During the initiation or induction stage, a hydrogen atom is extracted from the methyl ester backbone (by an initiator molecule, .I) producing a free radical, R.. In the presence of oxygen, a diradical itself, a radical peroxide molecule forms, ROO.. It is possible to form a hydroperoxide molecule, ROOH, if another hydrogen atom is extracted from an adjacent methyl ester again forming and additional methyl ester radical. Continuation of this propagation stage results in numerous hydroperoxides and methyl ester radicals leading to accelerated oxidative instability. This will continue until the dissolved oxygen is consumed. Termination follows with the formation of a simple hydrocarbon, R—R:

1. Initiation: RH+.I→R.+HI

2. Propagation: R.+O₂→ROO.ROO.+RH→ROOH+R.

3. Termination: R.+R.→R—R

There are two types of technology that may be used to increase oxidative stability of methyl esters, one focusing on Step 2 (use of antioxidants) and the other stopping Step 1 from progressing to Step 2. The use of antioxidants, as disclosed for example in US 2007/0137098 and WO 2007/146567, removes or lowers the concentration of dissolved oxygen in the methyl esters thus limiting propagation of hydroperoxide formation. With regard to US 2007/0137098 and WO 2007/146567, there are two issues to be concerned with: a) the use of a volatile antioxidant such as diethylhydroxylamine (DEHA) and b) the initiation step is allowed to progress uncontrollably. A typical oxygen scavenger such as DEHA will consume dissolved oxygen (DO). DEHA is oxidized in the process going through a series of sequential steps and in the process DO is reduced to water. The driving force for the decomposition of the hydroxylamine to the nitrone is the reduction of dissolved oxygen to water. The nitrone can then hydrolyze forming ethyhydroxylamine and acetaldehyde. With continued ingress of oxygen into the biodiesel (from opening and closing the storage tank), the DEHA and any phenolic antioxidant is quickly consumed leading to accelerated biodiesel instability. Even with the use of an oxygen scavenger, Step 1, initiation, is allowed to progress resulting in a large number of free methyl ester radicals, R.

As discussed in US 2007/0137098, it may be possible to use a precursor molecule to an oxygen scavenger such as DEHA. For example, triethylamine N-oxide (TEAO) decomposes slowly under typical ambient conditions to form ethylene and diethylhydroxylamine. This is viewed as an unwanted technology for two reasons. Since diethylhydroxylamine is volatile and may partition into the headspace of the storage vessel, it may be lost from continued opening and closing of the storage vessel. Replenishment of lost DEHA may come from excess TEAO. At some point in time with continued opening and closing of the storage container, all the TEAO will be converted to DEHA and eventually the DEHA itself will be oxidized as discussed above. Also, as mentioned, ethylene is a by-product of TEAO decomposition and as such, its concentration may build is a closed storage container possible leading to an unwanted, hazardous and explosive condition.

To obviate these drawbacks, the compositions according to the invention comprise at least one nitroxide free-radical scavenger or precursor thereof. Moreover, they comprise at least one alkylalkanolamine.

Nitroxide Free-Radical Scavenger

With the use of a stable nitroxide free-radical scavenger, oxidative instability is slowed or halted in Step 1 vs. Step 2. A nitroxide is already a free radical, R—O., and as soon as the methyl ester radicals form in Step 1, they are capped or stabilized by the nitroxide. The difference between using a hydroxylamine which produces an intermediate nitroxide and a molecule such as TEMPO is that the TEMPO is a stable nitroxide whereas the nitroxide produced from DEHA decomposition is unstable. Also, DEHA may be too volatile being rapidly lost from continued opening and closing of the storage vessel. TEMPO or other nitroxides are not nearly as volatile and therefore are not lost leading to a more stabilized fuel.

The use of stable nitroxide such as TEMPO (or its derivatives) thus results in a more stable biodiesel. It stops the oxidative instability after Step 1 (initiation) rather than slowing Step 2. By stopping decomposition at Step 1 using a stable nitroxide, the biodiesel is more tolerant to oxygen (and light) so the storage container may be opened and closed numerous times as opposed to keeping it air tight.

The nitroxide free-radical scavenger is a stable free radical having preferably the formula (I):

wherein R₁, R₂, R₃ and R₄ are alkyl groups or heteroatom substituted alkyl groups and no hydrogen is bound to the remaining valences on the carbon atoms bound to the nitrogen.

The alkyl (or heteroatom substituted) groups R₁-R₄ may be the same or different, and preferably contain 1 to 15 carbon atoms. Preferably R₁-R₄ are methyl, ethyl, or propyl groups. In addition to hydrogen the heteroatom substituents may include, halogen, oxygen, sulfur, nitrogen and the like.

The remaining valences (R₅ and R₆) in the formula above may be satisfied by any atom or group except hydrogen which can bond covalently to carbon, although some groups may reduce the stabilizing power of the nitroxide structure and are undesirable. Preferably R₅ and R₆ are halogen, cyano, —COOR wherein R is alkyl or aryl, —CONH₂, —S—C₆H₅, —S—COCH₃, —OCOC₂H₅, carbonyl, alkenyl where the double bond is not conjugated with the nitroxide moiety or alkyl of 1 to 15 carbon atoms, R₅ and R₆ may also form a ring of 4 or 5 carbon atoms and up to two heteroatoms, such as O, N or S by R₅ and R₆ together. Examples of suitable compounds having the structure above and in which R₅ and R₆ form part of the ring are pyrrolidin-1-oxys, piperidinyl-1-oxys, the morpholines and piperazines. Particular examples wherein the R₅ and R₆ above form part of a ring are 4-hydroxy-2,2,6,6-tetramethyl-piperidino-1-oxy (4-hydroxy TEMPO), 2,2,6,6-tetramethyl-piperidino-1-oxy (TEMPO), 4-oxo-2,2,6,6-tetramethyl-piperidino-1-oxy and pyrrolin-1-oxyl. Suitable R₅ and R₆ groups are methyl, ethyl, and propyl groups. A specific example of a suitable compound where R₁-R₆ are alkyl groups is di-tert-butylnitroxide. The preferred carbonyl containing nitroxides are those wherein the R₅ and R₆ form a ring structure with the nitrogen, preferably a six number ring, for example, 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy, which is the preferred nitroxide for use in the present invention.

The composition contains preferably at least 1 ppm, more preferably at least 5 ppm and most preferably at least 10 ppm of said nitroxide or nitroxides. Advantageously, it contains less than 200 ppm, preferably less than 100 ppm and more preferably less than 50 ppm of said nitroxide or nitroxides.

Alkylalkanolamine

The fatty ester compositions of the present invention contain at least one alkylalkanolamine. This alkylalkanolamine may comprise at least one alkylalkanolamine selected from the group consisting of N-alkylalkanolamines, N-alkyldialkanolamines and N-dialkylalkanolamines. The alkylalkanolamine may thus have the formula (II):

R¹R²NCH₂CH₂OH

wherein R¹ is an alkyl group or an isoalkyl group of 3 to 24 carbon atoms and R² is —H, —CH₂, —CH₂CH₂OH or —R¹.

The alkylalkanolamine is preferably selected from the group comprising butyldiethanolamine (BDEA), butylaminoethanol (BAE), dibutylaminoethanol (DBAE), diisopropylaminoethanol (DIPAE), octylaminoethanol (OAE) and octyldiethanolamine (ODEA).

The composition contains preferably at least 50 ppm, more preferably at least 100 ppm and most preferably at least 200 ppm of said alkylalkanolamine or alkylalkanolamines. The total alkylalkanolamine content is however preferably less than 10 000 ppm.

An important advantage of the use of an alkylalkanolamine is that it cannot only be used to improve the oxidative stability of the composition, but also to assist in dissolving the nitroxide free-radical scavenger. This nitroxide is soluble in the fatty ester composition but doesn't dissolve easily therein. It takes indeed a long time to dissolve the nitroxide therein or the fatty ester composition has first to be heated. It has however now been found that the nitroxide dissolves easily in the alkylalkanolamine so that it can first dissolved therein and the obtained solution can then be added easily to the fatty ester component. The nitroxide remains dissolved therein and no heating of the fatty ester component is required.

The composition may, aside from the nitroxide and the alkylalkanolamine, consist essentially of the biodiesel (and optionally the petroleum distillates), or it may also contain other optional additives such as those detailed below. It should be noted that certain additives, when included in the compositions of this invention, may have a substantial effect on important properties of the treated composition. The effects of such changes may or may not be desirable in a given situation, and therefore some embodiments of the invention preclude the use of certain additives in an amount that materially affects one or more of these properties. Examples of such additives whose presence (in high enough amounts) may be precluded include compounds known to accelerate atmospheric oxidation of unsaturated fatty acids and their esters, including for example cobalt and manganese driers such as are used for curing alkyds and drying oils. Generally, oxidizing agents should be avoided. Such materials might include hydrogen peroxide or organic peroxides.

As distinct from the foregoing list of additives, certain other additives may typically be included in the treated composition in an amount sufficient to achieve certain performance advantages. In the case where the composition comprises a biodiesel, conventional diesel additives may be included. For example, surfactants may be included to help reduce the build-up of deposits. Other ingredients might also include octane boosters, cetane enhancers, pour point or cloud point depressants, and explosion suppressors (e.g., tetraethyllead), and fatty acids for use as friction modifiers. Water may also be present in the treated fuel. If present, water may in some embodiments be included in only small amounts, i.e., at less than 2 wt % or even less than 0.5 wt %, most typically less than 500 ppm, as measured by ASTM 6751. It may however be present in larger amounts, for example from 2 to 25 wt % based on the total weight of the resulting mixture, more commonly 10 to 15 wt %, in the form of a solution, stabilized emulsion, or other dispersion.

The following optional additives may be used to further enhance the oxidative stability of the composition:

Nonphenolic Oxygen Scavenger

The nonphenolic oxygen scavenger may be a hydroxylamine. As explained hereabove, hydroxylamines are oxygen scavengers which can assist in improving the oxidative stability of the fatty ester composition.

Nonlimiting examples of suitable hydroxylamines are according to the formula R¹R²NOH, wherein R¹ and R² are each independently hydrogen, a linear or branched, saturated or unsaturated C1-C20 aliphatic moiety, which can optionally be mono- or polysubstituted, or a C6-C12 aryl moiety, a C7-C14 araliphatic moiety or a C5-C7 cycloaliphatic moiety. Representative hydroxylamines include but are not limited to: hydroxylamine, methylhydroxylamine, dimethylhydroxylamine, methylethylhydroxylamine, ethylhydroxylamine, diethylhydroxylamine (DEHA), dibutylhydroxylamine, dibenzylhydroxylamine, monoisopropylhydroxylamine and mixtures thereof.

The composition contains preferably at least 50 ppm, more preferably at least 100 ppm and most preferably at least 200 ppm of said hydroxylamine or hydroxylamines. The total hydroxylamine content is however preferably less than 10 000 ppm.

In the same way as the alkylalkanolamine, the hydroxylamine can also be used to dissolve the nitroxide free-radical scavenger into the fatty ester component. It has indeed been found that the nitroxide dissolves easily in the hydroxylamine so that it can first be dissolved therein and the obtained solution can then be added easily to the fatty ester component. The nitroxide remains dissolved therein and no heating of the fatty ester component is required. Instead of dissolving the nitroxide into the alkylalkanolamine or into the hydroxylamine, it can also be dissolved in a mixture thereof.

Another class of suitable nonphenolic oxygen scavengers comprises oximes derived from aldehydes or ketones. Examples include 2-butanone oxime, acetone oxime, cyclohexanone oxime, benzoin oxime, propanal oxime, butanal oxime, and isobutanal oxime.

Nitrones are also suitable for use as nonphenolic oxygen scavengers. Any nitrone may be used. Suitable classes of nitrones may be described according to the formula

wherein R₁ and R₂ may be the same or different and are each selected from the group consisting of hydrogen and hydrocarbon radicals having between one and ten carbon atoms. R₃ is a hydrocarbon radical having between one and ten carbon atoms. R₁, R₂, and R₃ may all be selected from alkyl groups (saturated or unsaturated), cycloalkyl groups, aryl groups, or aralkyl groups. Examples of suitable alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, and the various n-hexenyl, n-heptenyl, n-octenyl, n-nonenyl and n-decenyl radicals. Examples of cycloalkyl, aryl, and aralkyl groups, respectively, include cyclohexyl, phenyl, and tolyl radicals. Typically the hydrocarbon radicals are groups having from one to seven carbons. Specific examples of suitable nitrones include formaldehyde isopropylnitrone; formaldehyde ethylnitrone, formaldehyde methylnitrone, acetaldehyde isopropylnitrone, acetaldehyde propylnitrone, acetaldehyde ethylnitrone, acetaldehyde methylnitrone, acetone isopropylnitrone, acetone propylnitrone, acetone ethylnitrone, acetone methylnitrone, acetone n-butylnitrone, acetone benzylnitrone, formaldehyde n-hexylnitrone, methyl ethyl ketone ethylnitrone, formaldehyde cyclohexylnitrone, isobutyraldehyde isopropylnitrone, isobutyraldehyde ethylnitrone, n-butyraldehyde isoproylnitrone, n-butyraldehyde ethylnitrone, and n-butyraldehyde propylnitrone.

In some embodiments, the oxygen scavenger comprises a compound having a vapor pressure greater than 10 Torr at 25° C., preferably greater than 20 Torr, and more preferably greater than 30 Torr. Diethylhydroxylamine is an example of such a compound, having a vapor pressure of 32 Torr. The use of a sufficiently volatile scavenger may improve the efficacy of the antioxidant package by capturing oxygen in the headspace above the composition, thereby preventing at least some of the oxygen from reacting with unsaturated fatty esters. A drawback of such antioxidants is however that they may be more quickly lost. The nonphenolic oxygen scavenger (and/or precursor thereof) may be incorporated in the treated composition in any amount. Typically, it will be present in an amount equal to from 0.001 to 5 wt % relative to the fatty ester component, more typically from 0.01 to 2 wt %, and most typically from 0.01 to 1 wt %.

Precursors To Non-Phenolic Oxygen Scavengers

Precursors to certain non-phenolic oxygen scavengers, for example precursors to hydroxylamines, may be used in place of or in addition to the non-phenolic oxygen scavengers themselves. As used herein, the term “precursor” means a compound that liberates, or is converted to, the desired compound in the composition in an amount sufficient to provide resistance to oxidative degradation. In the case of hydroxylamine, one type of precursor is a salt thereof with an organic or inorganic acid. Such acids may include as nonlimiting examples hydrochloric acid, sulfuric acid, sulfonic acids, phosphonic acids, and carboxylic acids.

Another type of precursor for hydroxylamines is amine N-oxides. For example, triethylamine N-oxide decomposes slowly under typical ambient conditions to form ethylene and diethylhydroxylamine. Any N-oxide is suitable for use. In some applications, it is preferable that the N-oxide not act as a surfactant, for example in cases where another surfactant package is used or when no surfactant at all is desired. Some examples of amine N-oxide types that show little surfactancy include those of the formula R³R⁴R⁵N→O, in which R³, R⁴, and R⁵ are each individually selected from C1-C8 linear or branched alkyl groups, provided that at least one of R³, R⁴, and R⁵ has a primary, secondary, or tertiary carbon atom at the 2 position relative to N, so that the group may split out to form an olefin and thereby produce a hydroxylamine.

Phenolic Antioxidant or Precursor

Suitable phenolic antioxidants may be selected from a wide variety of materials known in the art. For example they may be substituted or unsubstituted hydroquinones. Nonlimiting examples include hydroquinones substituted in the ortho or meta positions (or both) with moieties including but not limited to C-1 to C-6 alkyl or aryl moieties. Two suitable examples are methylhydroquinone and tert-butylhydroquinone. In general, suitable phenolic antioxidants include any known dihydroxybenzene or aminohydroxybenzene compound or a lower alkyl, e.g., 1 to 8 carbon atoms, substituted derivative thereof. Specific suitable compounds include 2,4-diaminophenol; 5-methyl-o-aminophenol; o-aminophenol; p-aminophenol; 3-methyl-p-aminophenol; 4,6-diamino-2-methylphenol; p-methylaminophenol; m-aminophenol; p-(N-methylamino)phenol; o-(N-butylamino)phenol; 3,4-dihydroxybenzaldehyde; and 2,5-dihydroxybenzaldehyde. Other examples include catechols and substituted catechols, especially tertiary alkyl substituted ones. Some specific examples are p-(tert-butyl)catechol, p-(1,1-dimethylethyl)catechol, p-(1-ethyl-1-methyl hexyl)catechol, p-(1,1-diethylpropyl)catechol, p-tributylmethylcatechol, p-trihexylmethylcatechol, and p-(1,1-diethylethyl)catechol, etc. Precursors of phenolic antioxidants include benzoquinone and naphthoquinone, which may be converted to the corresponding phenolic compounds by contact with a reducing agent such as the nonphenolic oxygen scavenger. The phenolic antioxidant (and/or precursor thereof) may be incorporated in the treated composition in any amount. The effective amount of phenolic antioxidant may in some cases be as low as 0.01 ppm by weight, relative to the fatty ester component. Typically, it will be present in an amount equal to from 1 to 500 ppm, more typically from 2 to 200 ppm, and most typically from 4 to 100 ppm.

Treated compositions according to the invention generally provide low rates of oxidative degradation, making them suitable for use in a number of applications. They may for example be particularly suitable as biodiesel fuels for use in cold climates, where the negative effects of oxidative degradation are may be particularly troublesome due to the resulting increase in pour point temperature.

EXAMPLES Example 1 Solubility Tests

a) An antioxidant package was prepared by dissolving 0.5 grams of 4-hydroxy TEMPO (4-HT) into 10 grams of octylaminoethanol (OAE). The nitroxide was completely soluble in the alkanolamine.

b) An antioxidant package was prepared by dissolving 0.5 grams of 4-HT along with 1 gram propyl gallate into 15 grams OAE. The nitroxide and ester of gallic acid were completely soluble in the alkanolamine.

c) An antioxidant package was prepared by dissolving 0.5 grams of 4-HT into 7.5 grams anhydrous diethylhydroxylamine (DEHA). The nitroxide was completely soluble in the hydroxylamine.

d) An antioxidant package was prepared by dissolving 0.5 grams of 4-HT into 7.5 grams anhydrous DEHA. Additionally, 2 grams of octyldiethanolamine (ODEA) was added to the additive mixture. The nitroxide and amine was completely soluble in the hydroxylamine.

Example 2 Combination of ODEA or BDEA and 4-Hydroxy TEMPO

Use was made in this example of a pure soy-biodiesel (methyl ester of soy oil) from a commercial source having a Rancimat oxidative stability (tested in accordance with EN 14112) of 5.1 hours.

a) Addition of 50 ppm 4-HT increased the oxidative stability of this B100 biodiesel to 5.5 hours.

b) Addition of 450 ppm ODEA increased the oxidative stability of this B100 biodiesel to 8.3 hours.

c) Addition of 450 ppm ODEA plus 50 ppm 4-HT increased the oxidative stability test to 9.8 hours. Comparing Example 2a and Example 2b to this Example, it is easy to see the synergy that develops between ODEA and 4-HT.

d) Addition of 450 ppm BDEA to the soy methyl ester in Example 2a increased the oxidative stability test to 6.1 hours.

e) Addition of 475 ppm BDEA plus 25 ppm 4-HT to the soy methyl ester in Example 2a increased the oxidative stability test to 9.4 hours. Comparing Example 2a and Example 2d to this Example, it is easy to see the synergy that develops between BDEA and 4-HT.

f) Using a different sample of soy methyl ester, addition of 250 ppm BDEA resulted in an oxidation stability of only 3.5 hours. Addition of 250 ppm BDEA+12.5 ppm 4-HT increased the oxidative stability test to 6.0 hours

g) Using a different commercial sample of soy methyl ester, the oxidative stability was only about 5.3 hours. Addition of 95 ppm ODEA+5 ppm 4-HT increased the oxidative stability test to 9.8 hours. 

1. A composition comprising: a fatty ester component constituting at least 50 wt % of the composition and comprising unsaturated fatty esters; and at least one alkylalkanolamine, characterised in that the composition further comprises at least one nitroxide free-radical scavenger or a precursor thereof.
 2. The composition as claimed in claim 1, which comprises at least one alkylalkanolamine selected from the group consisting of N-alkylalkanolamines, N-alkyldialkanolamines and N-dialkylalkanolamines.
 3. The composition as claimed in claim 2, wherein said alkylalkanolamine has the formula (II): R¹R²NCH₂CH₂OH wherein R¹ is an alkyl group or an isoalkyl group of 3 to 24 carbon atoms and R² is —H, —CH₂, —CH₂CH₂OH or —R¹.
 4. The composition as claimed in claim 2, wherein said alkylalkanolamine is selected from the group comprising butyldiethanolamine (BDEA), butylaminoethanol (BAE), dibutylaminoethanol (DBAE), diisopropylaminoethanol (DIPAE), octylaminoethanol (OAE) and octyldiethanolamine (ODEA).
 5. The composition as claimed in claim 1, wherein the nitroxide free-radical scavenger has the formula (I):

wherein each of R₁, R₂ and R₃ and R₄ is an alkyl group or heteroatom substituted alkyl group having 1 to 15 carbon atoms, and wherein R₅ and R₆ (a) each being an alkyl group having 1 to 15 carbon atoms, or a substituted alkyl group having 1 to 15 carbon atoms wherein the substituent is halogen, cyano, —CONH₂, —SC₆H₅, —S—COCH₃, —OCOCH₃, —OCOC₂H₅, carbonyl, alkenyl wherein the double bond is not conjugated with the nitroxide moiety, or —COOR wherein R of the —COOR group is alkyl or aryl, or (b) together forming part of a ring that contains 4 or 5 carbon atoms and up to two heteratoms of O, N or S.
 6. The composition as claimed in claim 5, wherein the R₁, R₂, R₃ and R₄ groups in formula (I) are each methyl, ethyl or propyl groups.
 7. The composition as claimed in claim 5, wherein R₅ and R₆ in formula (I) are each methyl, ethyl or propyl groups.
 8. The composition as claimed in claim 1, wherein the nitroxide is a piperidino-1-oxyl, a pyrrolidino-1-oxyl or a pyrrolin-1-oxyl.
 9. The composition as claimed in claim 8, wherein the nitroxide is selected from the group consisting of 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy (4-hydroxy TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidino-1-oxy and 2,2,6,6-tetramethylpiperidino-1-oxy (TEMPO), the nitroxide comprising preferably 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy.
 10. The composition as claimed in claim 1, wherein the nitroxide is di-tert-butyl-nitroxide.
 11. The composition as claimed in claim 1, which contains at least 1 ppm, preferably at least 5 ppm and more preferably at least 10 ppm of said nitroxide.
 12. The composition as claimed in claim 1, which contains less than 200 ppm, preferably less than 100 ppm and more preferably less than 50 ppm of said nitroxide.
 13. The composition as claimed in claim 1, which contains at least 50 ppm, preferably at least 100 ppm and more preferably at least 200 ppm of said alkylalkanolamine, but preferably less than 10 000 ppm thereof.
 14. The composition as claimed in claim 1, wherein the fatty ester component comprises a biodiesel.
 15. The composition as claimed in 4, wherein the fatty ester component contains at least 10 wt %, preferably at least 25 wt % and more preferably at least 40 wt % of said unsaturated fatty esters.
 16. A method of making a stabilized composition as claimed in claim 1, comprising blending together: a fatty ester component comprising an unsaturated fatty ester; at least one alkylalkanolamine; and a nitroxide free-radical scavenger or precursor thereof.
 17. The method as claimed in claim 16, wherein said nitroxide free-radical scavenger is dissolved in said alkylalkanolamine before being blended with said fatty ester component.
 18. The method as claimed in claim 16, wherein said nitroxide free-radical scavenger is dissolved in a hydroxylamine, or in a mixture of a hydroxylamine and said alkylalkanolamine, before being blended with said fatty ester component. 